1. Field of the Invention
The present invention relates to a reproducing circuit of a wobble signal recorded on a recordable disk of an optical disk.
2. Description of the Related Art
At present, a write once optical disk such as a CD-R/DVD-R, and a recordable optical disk such as a CD-RW/DVD-RW or DVD-RAM, in addition to a read-only optical disk such as a CD or a DVD-RAM, are put on the market. At the time of recording of these recordable optical disks, clock signal generation from a reproduction signal, which is performed in a conventional read-only disk, can not be performed. Thus, a groove as a portion where information is recorded on a disk is wobbled at a constant period, and a clock signal at the time of recording is generated from a wobble signal obtained by reproducing this wobbling (hereinafter referred to as wobble) by a push-pull system.
FIG. 2 shows a conventional example of a wobble signal reproducing circuit. Reflected light from a laser spot irradiated on a disk forms a far field pattern 2 on an optical detector 1 divided in two. Two outputs of the optical detector are subtracted by a subtracting circuit 17 to take out a push-pull component. From the obtained push-pull signal, a wobble frequency is extracted by a band pass filter (BPF) 18. A wobble signal reproduced from a CD-R/RW is a continuous signal of a frequency of 22.05 KHz±1 KHz. In the CD-R/RW, since address information called ATIP (Absolute Time In Pregroove) is superimposed on the wobble signal by FM modulation, it has a width in frequency band. FIG. 3 is a view showing a disk surface of a DVD-R/RW. Reference numeral 301 in the drawing designates a groove in which data is recorded and which is wobbled similarly to the CD-R/RW. A wobble signal reproduced in the DVD-R/RW is a continuous signal of a single frequency of 140.65 KHz different from the CD-R/RW, and address information is not superimposed. Instead thereof, a pit called a land pre-pit (hereinafter referred to as an LPP) designated by 302 is recorded in a land portion which is between a groove and a groove and in which information is not recorded. The LPP exists in a region from the head of maximum amplitude position of a wobble to the third, and address information is recorded by this three bits.
In order to stably extract the wobble signal from the push-pull signal, it becomes necessary to reduce leakage of an RF signal to the push-pull signal. Thus, as disclosed in Japanese Patent Unexamined Publication No. Hei. 8-194969, it is conceivable that an automatic gain control circuit (AGC circuit) is provided at a front stage of the subtracting circuit of FIG. 2.
Besides, in recent years, as the capacity of a disk is increased, doubling of the speed of disk reproduction becomes active.
In double speed reproduction, it becomes necessary to change over the pass band of the BPF of wobble in connection with the double speed. As a method of causing this connection, although it is conceivable that setting of the pass band of the BPF is changed over by a microcomputer according to the reproducing speed of a disk, since access to the microcomputer becomes necessary each time the speed is changed, the processing becomes troublesome. In the case where CAV reproduction is performed with an increase in speed of disk reproduction, a deviation in band setting of the BPF occurs by the restriction on the number of accesses to the microcomputer, and this becomes a factor of a drop in S/N of the wobble signal and an increase in clock jitter due to that. With respect to this problem, as disclosed in Japanese Patent Unexamined Publication No. Hei. 11-203681, it is conceivable to use pass band automatic tracking by a dummy filter and a phase comparison circuit.